Sorting Array of objects

Lost User

See this article. You need to implement the IComparable interface and add a function CompareTo to your class. regards

Rizwan Rathore

Hi Sir, Well the article u have suggested is quite complex coz i think it is about sorting objects of different classes.....i just need to sort object of a single class on the base of a specific data member........isnt there any easy way to do this :( Looking forward for help Regards,

Robert Rohde

You should also have read his second sentence. Here a little example:

public class MyClass : IComparable {
private string _myValue;

public int CompareTo(object o) {
return _myValue.CompareTo(((MyClass)o)._myValue);
}
}

BoneSoft

using System;
using System.Collections;

namespace Test {
// your class
public class Word {
private string value;
private string anothervalue;

	public string Value {
		get { return value; }
		set { value = value; }
	}

	public string AnotherValue {
		get { return anothervalue; }
		set { anothervalue = value; }
	}
}

public class WordComparer : IComparer {
	public int Compare(object x, object y) {
		Word w1 = x as Word;
		Word w2 = y as Word;
		if (w1 == null || w2 == null) { return 0; }
		return w1.Value.CompareTo(w2.Word);
	}
}

// your application
public class Application {
	public Word\[\] Sort(Word\[\] words) {
		return Array.Sort(words, new WordComparer());
	}
}

}

Rizwan Rathore

how can i call this in my class? and wat is the purpose of this _myValue variable. looking forward for help Regards, -- modified at 11:59 Tuesday 23rd May, 2006

Robert Rohde

Ok some more details :): MyClass is the class you want to get sorted. _myValue is the field or whatever by which you want to sort the data. Instead of using a field you could also use a property or something else. Now assuming you have filled instances of this class into an ArrayList you can sort it with:

list.Sort();

If you have an array (MyClass[] myArray) of your object than you can sort it with:

Array.Sort(myArray);

Internally the Sort methods will call CompareTo to determine the correct ordering.

Rizwan Rathore

Hi Sir, when i am trying to do this in this way

public class WordComparer : IComparer
{

public int Compare(object x, object y) 
{			
	Terminology w1 = x as Terminology;			
	Terminology w2 = y as Terminology;			
	if (w1 == null || w2 == null) { return 0; }		
	return w1.term.CompareTo(w2.term);		
}

}

public class TextMiner : System.Windows.Forms.Form
{
some code here........
public Terminology[] word = new Terminology[WORDS];
public void Indexer()
{
Array.Sort(word, new WordComparer());
}
}

well it compiles fine but when i try to access the objects of that sorted class it gives null object reference exception

for(int i = 0; i < Terminology.Count; i ++)
{
do something with word[i];
}

well this exception comes when the value of i is 0......and the program was working perfectly fine b4 that..... Looking forward for help Regards,

Rizwan Rathore

Hi Sir, i am trying to do this in this way

public class TextMiner : System.Windows.Forms.Form , IComparable
{
public Terminology[] word = new Terminology[WORDS];
private string strValue = "term";
public int CompareTo(object obj)
{
Terminology w = (Terminology) obj;
return( strValue.CompareTo( w.term ));
}

     public void Indexer()
     {
             Aray.Sort(word);
     }

}

it compiles without any error but it throws this exception during execution An unhandled exception of type 'System.InvalidOperationException' occurred in mscorlib.dll Additional information: Specified IComparer threw an exception. tell me what i m doing wrong and how i can correct it.... looking forward for help Regards,

BoneSoft

I can't say much about the exception without seeing how you're setting up the arrays... But from your code snippet above, Array.Sort returns a new array. Try:

public class TextMiner : System.Windows.Forms.Form {
//some code here........
public Terminology[] word = new Terminology[WORDS];
public void Indexer() {
word = Array.Sort(word, new WordComparer());
}
}

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Robert Rohde

BoneSoft wrote:

Array.Sort returns a new array

No it doesn't - it sorts the existing instance

Robert Rohde

As you sort a Terminology array also the Terminology class should implement IComparable and not the Form.

Rizwan Rathore

BoneSoft wrote:

word = Array.Sort(word, new WordComparer());

well i ve tried this already but it gives compile time error that cant covert type void to type TextMiner.Terminology[] :(

Rizwan Rathore

sir it also didnt work :( i ve written the same code in the CompareTo function which i ve written in TextMiner class but it made no difference again same exception :( is there any way out of this ?? looking forward for help Regards,

Robert Rohde

I assume some error is thrown within your CompareTo method. You should probably check if the are null values within your array or probably if sometimes term is null.

Rizwan Rathore

well the exception doesnt come during sorting it comes after the sorting the statement i m cofused about is the use of private string strValue = "term"; isnt there any error in this statement?? i mean term is the variable of terminology class and i m declaring it as a string value in TextMiner class how will this work??

Robert Rohde

Could you please post the important part of the Terminology class and the Sort call you are currently doing? Maybe I can help you then.

Rizwan Rathore

public class Terminology : IComparable
{
private string term;
private int df;
private int qTf;
private double qWeight;
private ArrayList tf = new ArrayList();
private ArrayList position= new ArrayList();
private ArrayList docID= new ArrayList();
private static int Count;

    public int CompareTo(object obj)
{
	Terminology w = (Terminology) obj;
	return( w.term.CompareTo( w.term ));
}

//only method in this class other than properties and constructors
public void incDF()
{
df++;
}
}

now moving to TextMiner class

public class TextMiner : System.Windows.Forms.Form , IComparable
{
const int WORDS = 20000;
public Terminology[] word = new Terminology[WORDS];
public void Indexer()
{
Array.Sort(word);
}
private string strValue = "term";
public int CompareTo(object obj)
{
Terminology w = (Terminology) obj;
return( strValue.CompareTo( w.term ));
}
}

it gives this exception An unhandled exception of type 'System.NullReferenceException' occurred in 2ndAssignment.exe Additional information: Object reference not set to an instance of an object. and on this statement

for(int j = 0; j < Terminology.count-1; j++)
{
sw.Write(j+1 +"\t"+word[j].term.PadRight(20,' '));
}

and the exceptions comes when the value of j is 0....m really confused :( looking forward for help Regards, -- modified at 5:43 Wednesday 24th May, 2006

Robert Rohde

Several things: 1. In the first snippet the CompareTo should look like the following:

public int CompareTo(object obj)
{
Terminology w = (Terminology) obj;
return( this.term.CompareTo( w.term ));
}

In your implementation you compared w.term with itself. 2. The following part isn't needed (as far as I can see from your snippets):

private string strValue = "term";
public int CompareTo(object obj)
{
Terminology w = (Terminology) obj;
return( strValue.CompareTo( w.term ));
}
}

3. If you are getting such an error when j is 0 than my previous assumption that there are null values in your array seems right. The first element of the word array seems to be null. You could verify this for example with the following code:

for(int j = 0; j < Terminology.count-1; j++)
{
if (word[j] == null)
MessageBox.Show("word array at index " + j + " is null");
else if (word[j].term == null)
MessageBox.Show("term property in word array at index " + j + " is null");
sw.Write(j+1 +"\t"+word[j].term.PadRight(20,' '));
}

Rizwan Rathore

Robert Rohde wrote:

if (word[j] == null) MessageBox.Show("word array at index " + j + " is null");

Sir it is showing entire array of word as null.....but it works perfectly fine without sorting..........how can all the values changed to null??

BoneSoft

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